Commercial developments in automotive braking

Coronavirus has reduced 2020 automotive market sales, with an estimated 20-25% fewer vehicles sold overall. Internal Combustion Engine (ICE) car sales have also been hit by a much faster increase in sales of electric and hybrid cars than was anticipated. We are therefore seeing a higher percentage of cars relying on regenerative braking.

Regenerative braking gives Electric and Hybrid Vehicles a different braking pattern, with their brake pads being primarily used for emergency stops, decelerating the car from a low speed to a standstill (when it doesn’t have enough kinetic energy to regenerate), and as a backup in case of a power failure.

Friction braking will of course continue to dominate, and it will be years before automotive brake pads actually become redundant. Because of the changed braking behaviour, though, cars will brake less frequently and that will impact the material properties of both pad and rotor.

First, pads are likely to change in shape. The brake surface area should increase because larger batteries mean heavier cars. And the pads should become thinner to reduce weight, which is possible since there’s less wear.

Noise is becoming more important, which is a challenge for brake pad and rotor manufacturers. This is more of an issue with electric vehicle braking because even relatively quiet brake noises are audible from EVs since they are almost silent.

Corrosion on rotor and pad will become more important too. With ICE vehicles, regular brake use cleans any corrosion. With EV drivers using friction brakes less often and with less brake pressure, rust can build up on the rotor and brake pad surface. This can seriously impact the braking performance and create a safety issue. Applying the friction brakes several times can clean rotor and pad surfaces.

Braking will focus less on brake pedal ‘feel’ because an increasing number of cars will integrate electronically controlled brake boosters in which pedal feel is managed by software rather than the pad’s direct response.

We are also seeing something of a return to drum brakes on the rear axles of cars.

Disc brakes have over the last 70 years almost totally replaced drum brakes. Disc brakes provide much more braking power than drum brakes and, as most (>75%) of the braking energy comes from the front brakes, rear axles can use drum brakes.

The drum market is expected to grow particularly with the growth of both conventional and electric small city cars. Drum brakes cost less, and so make the car more affordable. Drivers do however prefer the look of shiny rotors and brake pads more than drum brakes, especially with very open rims. But since small city cars are often meant to be affordable, less money will be spent on open rims so they won’t be seen anyway.

Drum brakes are sealed and therefore do not suffer from rainwater ingress, so they also don’t suffer from corrosion. Enclosing the brakes in the drum also reduces particle emissions. Another advantage to drum brakes is the lower risk of stiction on the parking brake.

With Volkswagen using drum brakes on their all-electric MEB platform (also used by Ford) we foresee a comeback of drum brakes.

With an average 70,000 km (45,000 miles) life span, brake pads can be replaced close to three times on ICE driven passenger cars. Hybrid cars will on average change their brake pads once, and full electric vehicles should need no replacement at all. This will mean a tough time for manufacturers of aftermarket brake pads and the garages that fit them. By 2030, around 50% of all cars sold will still have a combustion engine and will therefore need replacement pads. By say 2035 the aftermarket will therefore be a lot smaller than it is today (although older ICE cars will still need replacement brake pads as maintenance items). HV and EV replacements, though, will most likely only be needed because of accidental damage.

Brake pad and rotor developers are now working on parts that are more corrosion resistant and/or can be cleaned more effectively. New and aftermarket brake pads alike will also need these higher performance, more stable, high-quality parts. And that means careful materials selection and higher investment in R&D.